The present invention relates to an optical element for use in an optical device utilizing the Faraday effect, and more particularly to a polarization independent optical isolator, an optical circulator, and a polarization beam combiner that are small and easy to assemble and require a very small number of components.
FIG. 25 illustrates an optical circulator disclosed in Japanese Patent No. 2539563. The circulator includes an optical collimator that includes optical fibers 12 and 14 fixedly mounted to Ferrules 11 and 20, respectively, and lenses 22 and 23, and another optical collimator that includes an optical fiber 82 fixedly mounted to a Ferrule 81 and a lens 72. There are provided two birefringent blocks 21 and 71 for branching a light beam into two optical paths or combining light beams into one optical path depending on the direction of travel and the polarization direction of the incident light beam(s). There are also provided a pair of wave plates 37 and 38 that cause the light to be polarized in the same direction. The wave plates 37 and 38 are positioned in parallel with respect to the light path. Positioned adjacent the wave plates 37 and 38 is a Faraday rotator 31 to which a magnet 32 applies an external magnetic field. Wave plates 67 and 68 are also positioned in parallel with resect to the light path. A Faraday rotator 61 is positioned adjacent the wave plates 67 and 68, and a magnet 62 applies an external magnetic field to the Faraday rotator 61. A birefringent block 41 is positioned in the middle of the optical circulator, so that the light passes through different optical paths depending on the direction of travel and the direction of polarization.
FIG. 26 illustrates another optical circulator disclosed in Japanese Patent No. 2539563. The optical circulator includes an optical collimator that includes optical fibers 12 and 14 fixedly mounted to Ferrules 11 and 20, respectively, and lenses 22 and 23, and another optical collimator that includes an optical fiber 82 fixedly mounted to a Ferrule 81 and a lens 72. There are provided two birefringent blocks 21 and 71 between the two optical collimators, the birefringent blocks branching a light beam into two optical paths or combining light beams into one optical path depending on the direction of travel and polarization direction of the light beam(s). Provided between the two birefringent blocks 21 and 71 are a pair of Faraday rotators 31 and 34 and a pair of Faraday rotators 61 and 64. The Faraday rotators 31 and 34 are positioned in parallel with respect to the light path. Permanent magnets 32 and 35 apply external magnetic fields to the pair of Faraday rotators 31 and 34 so that the polarization plane of the light passing through the Faraday rotators 31 and 34 is rotated by 45 degrees. The Faraday rotators 61 and 64 are positioned in parallel with respect to the light path. Permanent magnets 62 and 65 apply external magnetic fields to the pair of Faraday rotators 61 and 64 so that the polarization plane of the light passing through the Faraday rotators 61 and 64 is rotated by 45 degrees. A birefringent block 41 is positioned in the middle of the optical circulator, so that the light passes through different optical paths depending on the direction of travel and the direction of polarization of the light.
The optical circulator proposed in Japanese Patent No. 2539563 uses an optical collimator that is a combination of an optical fiber and a lens provided at each port. Therefore, individual optical components are of large sizes and therefore are not economical.
U.S. Pat. No. 5,991,076 in FIG. 27 and U.S. Pat. No. 6,049,426 in FIG. 28 propose miniaturized optical circulators in which the optical components can be small. In these optical circulators, a birefringent block 21 branches light emitted from an optical fiber 12 into two light beams having polarization planes perpendicular to each other, or combines two light beams having polarization planes perpendicular to each other into a single beam. A birefringent block 71 branches the light emitted from an optical fiber 82 into two light beams having polarization planes at right angles with each other, or combines two light beams having polarization planes perpendicular to each other into a single beam.
Referring to FIG. 27, a pair of wave plates 37-38, a pair of wave plates 67-68, and Faraday rotators 31 and 61 are arranged between two birefringent blocks 21 and 71. Each of the two pairs of wave plates 37-38 and 67-68 is positioned in parallel with respect to the light path and causes the light to be polarized in the same direction. Birefringent blocks 41 and 42 and lenses 51 and 52 are positioned symmetrically about the longitudinal middle of the optical circulator, so that the light passes through different light paths depending on the direction of travel of the light and the direction of polarization plane of the light.
The optical circulator in FIG. 27 allows optical fibers to be positioned irrespective of the shape of a lens, thus lending itself to the miniaturizing of optical circulators. However, the requirement of positioning wave plates in parallel with respect to the optical path places certain limitations on the miniaturization of optical circulators. Wave plates used in these optical devices usually have a size of about several millimeters square and are cut into desired sizes by means of, for example, a dicing saw. However, the use of a dicing saw causes chipping in the range of several microns to several tens microns at cut surfaces and edges of the wave plates. When light passes through the chipped portions, the optical characteristics of the wave plate deteriorate. Thus, it is required to ensure that light paths are separated by at least a certain distance in designing an optical circulator. This is a barrier to the miniaturization of circulators. In addition, the number of optical parts is 12, which is another factor behind increases in manufacturing cost.
FIG. 28 illustrates an optical circulator proposed in U.S. Pat. No. 6,049,426. This optical circulator includes two pairs of Faraday rotators, i.e., rotators 31 and 34 and rotators 61 and 64, which retain their magnetization and do not require permanent magnets. The Faraday rotators 31 and 34 are positioned in parallel with respect to the optical path. The Faraday rotators 61 and 64 are also positioned in parallel with respect to the optical path. Wollaston prisms 45 and 46 are used to form oblique light paths so that lenses 22 and 72 are shared by two light beams, thereby configuring an optical circulator in which miniaturized optical components can be used.
The optical circulator in FIG. 28 also requires two pairs of Faraday rotators, each pair including two Faraday rotators (31 and 34, or 61 and 64) aligned in parallel with respect to the light path. Therefore, the mechanical structure of the optical circulator necessarily places limitations on the miniaturization of an optical circulator. Moreover, the inventors has found that because a Faraday rotator is a ferrimagnetic material, when two Faraday rotators having opposite magnetization directions are positioned in contact with each other, the magnetic characteristics of the two Faraday rotators can affect each other to cause them to be demagnetized. Thus, the structure in FIG. 28 requires the two Faraday rotators to be somewhat spaced. This requirement implies that the light beams must further spaced apart.
In addition, the configuration in FIG. 28 can incorporate Faraday rotators only of the magnetization retaining type that requires no external magnetic field. The temperature and wavelength dependencies of Faraday rotation determine the performance of an optical device that uses a Faraday rotator. Commercially available Faraday rotators of the magnetization retaining type are prominently poor in temperature and wavelength dependencies as compared with ordinary Faraday rotators that are used in combination with magnets. Thus, optical circulators that employ Faraday rotators of the magnetization retaining type are inherently poor in temperature and wavelength dependencies as compared with optical circulators of other configuration.
The optical circulator in FIG. 28 incorporates a Wollaston prism formed of two birefringent blocks. The number of optical components is 12 and therefore it is essential to reduce the number of expensive optical components for lowering manufacturing costs.
In recent years, polarization dependent loss (PDL) has become an important characteristic of an optical device. PDL is a loss resulting from the difference in the polarization direction of light incident on an optical device. Optical devices including optical circulators, which utilize the polarization of light, are usually designed to have two light paths by using, for example, a birefringent block that branches the incident light into two light beams having polarization directions perpendicular to each other. When the two light beams travel in their corresponding paths, if the two beams transmit through different optical components, PDL results from differences in the insertion loss and characteristics of optical components. All of the aforementioned conventional optical circulators are of the configuration that optical components are aligned in parallel with respect to the light paths and therefore individual light beams pass through separate optical components. Such a configuration is apt to cause PDL inherently. Generally, variations in the insertion loss of a Faraday rotator are substantially in the range of a typical value xc2x10.02 dB. This implies that when light beams pass through a pair of optical components positioned in parallel in the light path, the light beams are subject to a PDL of a maximum of 0.04 dB.
Just as in optical circulators, there have also been proposed some configurations to polarization independent optical isolators. One such common apparatus is a combination of three birefringent blocks disclosed in Japanese Patents No. 58-28561 and No. 60-51690.
The polarization independent optical isolators disclosed in Japanese Patents No. 58-28561 and No. 60-51690 have a maximum isolation of about 40 dB, which is determined by the extinction ratio of the Faraday rotator. The Faraday rotation angle of a Faraday rotator depends on the temperature of the rotator and the wavelength of the light that passes the rotator. Therefore, changes in environmental temperature and laser wavelength cause serious deterioration of isolation. For high isolation, Japanese Patents No. 2539563 and 60-51690, 58-28561, and European Patent No. 0352002 have proposed, for example, the use of two polarization independent optical isolators positioned in series in the light path. This approach requires a large number of components and therefore uneconomical.
The inventors continued to research and study to develop a polarization independent optical isolator that has a small optical loss, can be assembled and adjusted easily, and requires a smaller number of components. As a result, the inventors proposed a configuration of a polarization independent optical isolator disclosed in U.S. Pat. No. 5,345,329. This polarization independent optical isolator has a feature of employing a double divided-domain Faraday rotator. The term double divided-domain Faraday rotator is used to cover a Faraday rotator that is made of a bismuth-substituted rare earth iron garnet single crystal film and is divided into two areas of a single domain structure when the Faraday rotator is in two external magnetic fields. When one of two adjacent areas of a Faraday rotator is placed in a magnetic field and the other of the adjacent areas is placed in another magnetic field, the two magnetic fields being opposite in direction, the two areas form magnetic domains having opposite magnetization directions. Thus, the two adjacent magnetic domains are bounded by an intermediate portion and exhibit Faraday rotations of opposite directions.
The configuration disclosed in U.S. Pat. No. 5,345,329 allows manufacture of a polarization independent optical isolator that requires a smaller number of components, can be assembled and adjusted easily, and is inherently free from PDL.
In practicing the configuration, the inventors used an ordinary optical collimator in which a lens is firmly attached to an end of an optical fiber. The diameter of commercially available lenses and optical collimators is 2 mm or larger. Because the beam diameter of collimated light is about 0.4 mm, the distance between light beams is 1.6 mm. Assuming that assembly margin is about 0.4 mm, it is only necessary for areas on a Faraday rotator 1 mm away from the boundary between two adjacent magnetic domains to function as double domains. However, in recent years, continuing miniaturization of optical components, development of lenses and optical collimators having small diameters, and development of dual-fiber Ferrules allow a single lens to be shared by light beams emitted from more than one optical fibers. This has led to miniaturized optical circulators as shown in FIGS. 27 and 28. The advent of such small optical devices has made it possible to design optical devices having a small distance between light beams and brought about the need for a reliable function of the inventors"" double domain Faraday rotator when light beams are very closely spaced.
Optical circulators and optical isolators according to the present invention have a feature that light emitted from an optical fiber is branched by a birefringent block and then directed to a double divided-domain Faraday rotator. For example, when a birefringent block is made of a rutile single crystal, the distance xcex94DB between the light beams branched by the birefringent block is given by Equation (4) as follows:       Δ    ⁢          xe2x80x83        ⁢          D      B        =                    {                                                            n                e                2                            -                              n                0                2                                                    2              ⁢                              n                e                            ⁢                              n                0                                              -                                    (                                                1                                      n                    0                                                  +                                  1                                                            n                      e                                        ⁢                                          cos                      ⁡                                              (                        σ                        )                                                                                                        )                        ⁢                          tan              ⁡                              (                θ                )                                                    }            xc3x97      L        =          0.0215      xc3x97      L      
where ne and no are refraction indexes and L is a length of the birefringent block.
For commercially available rutile single crystals, ne=2.452 and no=2.709 at a wavelength of 1550 nm. "sgr" represents a separation angle by which the light is branched by the birefringent block and is calculated by tan("sgr")=(ne2xe2x88x92no2)/2/ne/no=0.0998. xcex8 is calculated from a numerical aperture of a single mode optical fiber by NA=sin (xcex8). The numerical aperture of a single mode optical fiber is 0.1 and therefore the right side of Equation (4) is 0.0215L.
For a rutile single crystal, if the crystal is to be miniaturized such that the length of the crystal is half (i.e., about 4 mm) of a conventional length, then the distance between two light beams is 0.0215xc3x974=0.083 mm. Taking assembly errors into account, it is sufficient to ensure that the two magnetic domains are not apart from each other by more than 0.083 mm and are adjacent to each other with a boundary area having a maximum width of 0.04 mm between the two magnetic domains. In other words, the narrower the boundary region of a double divided-domain Faraday rotator that does not function as a Faraday rotator, the smaller the birefringent block can be made. This implies that if the machining accuracy and assembly accuracy of the optical components can be improved further, a rutile single crystal having a length less than 1 mm can be used to configure very small optical circulators and optical isolators. Such optical circulators and optical isolators can work with light beams that are separated by less than 0.02 mm.
As described above, with the double divided-domain Faraday rotator proposed in U.S. Pat. No. 5,345,329, it is highly desirable that the isolator operates properly and can work with light beams spaced apart by a short distance. If a double domain rotator is to be configured by placing permanent magnets beside the Faraday rotator, an important factor is a rate of change in magnetic field (denoted by DH in this specification) of the permanent magnets. The rate of change in magnetic field of the permanent magnets is usually about 3 cmxe2x88x921. For example, when a commercially available Faraday rotator having a small saturation magnetic field (e.g., 300 Oe) is used, external magnetic fields of +300 Oe and xe2x88x92300 Oe are not strong enough to cause the Faraday rotator to saturate. This implies that if the permanent magnets have a residual flux density of 10000 Gauss, then a boundary area of about 0.2 mm wide cannot function normally as a Faraday rotator. Likewise, when a Faraday rotator having a saturation magnetic field of 650 Oe, which exhibits good temperature dependence, is employed, a boundary area of about 0.43 mm wide cannot function normally as a Faraday rotator. Assuming an assembly error of 0.1 mm, the distance between light beams must be at least 0.53 mm, which is too long a distance to meet the requirement that optical isolator can operate when the distance between the light beams is very short.
The inventors researched and studied a double divided-domain Faraday rotator to develop polarization independent optical isolators, optical circulators, and polarization beam combiners that do not require two optical components to be positioned in parallel with respect to the light path. Such isolators, circulators, and beam combiners can be miniaturized to their theoretical limitations, have an inexpensive configuration, require a smaller number of components, and is difficult to cause PDL. As a result of their research and study, the inventors made the present invention. The present invention is based on the concept of the double divided-domain Faraday rotator proposed by the inventions in U.S. Pat. No. 5,345,329. When a double divided-domain Faraday rotator is used to make smaller-size optical circulators and polarization beam combiners, it is necessary to solve some technical problems. The inventors solved the technical problems, thereby proposing optical devices such as optical circulators, optical isolators, and polarization beam combiners that use a Faraday rotator.
A first problem is that when light passes the boundary area between adjacent magnetic domains of a double divided-domain Faraday rotator, a required Faraday rotation of the light cannot be obtained. Therefore, the light path must be designed such that the light does not pass through the boundary area between the adjacent magnetic domains. It is relatively easy to design a boundary area having a width of about 1 mm. However, it is required to design the boundary area having a width less than 0.1 mm for further miniaturization of isolators, circulators, and beam combiners.
A second problem is the drawbacks associated with the use of an optical collimator. For an optical collimator that is a combination of an optical fiber and a lens, there is a limit to the miniaturization of the lens and therefore the collimator for polarization independent optical isolators, optical circulators, and polarization beam combiners cannot be smaller beyond a certain size. For example, when an optical component having a size of 1 mm forms one port of an optical isolator, if a multi port optical isolator is to be designed using a commercially available 1 mm-diameter lens, the size of the multi port isolator will increase by 1 mm for each additional port. As described above, a commercially available optical collimator usually produces parallel light having a diameter of about 400 xcexcm. Thus, a birefringent block must branch light into two beams such that the center-to-center distance between the two beams is longer than 400 xcexcm. Thus, the second problem is to design a collimator in which the size and performance of a lens are not obstacles to miniaturization of isolators, circulators, and beam combiners.
The inventors made considerable efforts to make a narrow boundary area between adjacent two magnetic domains of a Faraday rotator, thereby implementing optical circulators, optical isolators, and polarization beam combiners that are small, inexpensive, and suitable for developing a multi port device.
An object of the invention is to provide a miniaturized Faraday rotator useful for optical circulators, optical isolators, and polarization beam combiners that are small, require a small number of optical components, and have multiple ports.
Another object of the invention is to provide these optical devices at low cost on a commercial basis in a large quantity, thereby contributing to the implementation and proliferation of optical communications.
The invention is directed to an optical device includes a Faraday rotator made of a bismuth substituted rare earth iron garnet single crystal film having a Faraday rotation of 45 degrees, and permanent magnets arranged beside the Faraday rotator to define two or more than two areas of a single domain structure in the Faraday rotator. Adjacent areas are magnetized in opposite directions to cause polarization planes of light beams passing through the adjacent areas to rotate in opposite directions. The optical device satisfies in the relation expressed by equation (1)
Hs/Br/DH greater than xcex94D/2 greater than 0xe2x80x83xe2x80x83(1)
where Hs (Oe) is a saturation magnetic field of the bismuth substituted rare earth iron garnet single crystal film, DH (cmxe2x88x921) is a rate of change in magnetic field in the proximity to a boundary between the adjacent areas, Br (Gauss) is a residual flux density of the permanent magnets, and xcex94D is a distance between the two light beams.
The Faraday rotator and the permanent magnets are in the relation expressed by equation (2)
(Dw*/H*)/(1/Br/DH)xe2x80x83xe2x80x83(2)
where H* is a lower limit of the magnetic field at which a width of a magnetic domain in a peripheral portion of the Faraday rotator starts to change more rapidly than a width of a magnetic domain in a middle portion of the Faraday rotator, and Dw* is a width of magnetic domain when the magnetic field is H*.
The Faraday rotator and permanent magnets are in the relation expressed by equation (3)
xe2x88x920.0000172Hs+0.0312 greater than (Hs/Br/DH)xe2x80x83xe2x80x83(3)
The permanent magnets include two permanent magnets arranged with the Faraday rotator positioned therebetween. Each of the two permanent magnets has a residual magnetic flux density Br in the range of 9000 to 12000 (Gauss). The two permanent magnets are spaced apart by a distance in the range of 0.08 to 0.18 (cm) so that DH is in the range of 2 to 5 (cmxe2x88x921). xcex94D is in the range of 0.005 to 0.05 (cm). The Faraday rotator has a saturation magnetic field equal to or less than 750 (Oe).
The permanent magnets may include at least three permanent magnets each of which has a residual magnetic flux density Br in the range of 9000 to 12000 (Gauss). The at least three magnets are aligned beside at least one side of the Faraday rotator at intervals in the range of 0.06 to 0.10 (cm) so that DH is in the range of 0.7 to 1.2 (cmxe2x88x921). xcex94D is in the range of 0.005 to 0.05 (cm) and the Faraday rotator has a saturation magnetic field equal to or less than 300 (Oe).
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.